There is no benefit in splitting the 32-bit address space into two distinct DMA zones when the 30-bit address space isn't even available on a device. If that is the case, default to one big ZONE_DMA spanning the whole 32-bit address space.
This will help reduce some of the issues we've seen with big crash kernel allocations. Signed-off-by: Nicolas Saenz Julienne <[email protected]> --- Whith this patch, on a 8GB RPi4 the setup looks like this: DMA [mem 0x0000000000000000-0x000000003fffffff] DMA32 [mem 0x0000000040000000-0x00000000ffffffff] Normal [mem 0x0000000100000000-0x00000001ffffffff] And stock 8GB virtme/qemu: DMA [mem 0x0000000040000000-0x00000000ffffffff] DMA32 empty Normal [mem 0x0000000100000000-0x000000023fffffff] arch/arm64/mm/init.c | 29 +++++++++++++++++++++++++---- 1 file changed, 25 insertions(+), 4 deletions(-) diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c index b6881d61b818..857a62611d7a 100644 --- a/arch/arm64/mm/init.c +++ b/arch/arm64/mm/init.c @@ -183,13 +183,20 @@ static void __init reserve_elfcorehdr(void) /* * Return the maximum physical address for a zone with a given address size - * limit. It currently assumes that for memory starting above 4G, 32-bit - * devices will use a DMA offset. + * limit or zero if memory starts from an address higher than the zone targeted. + * It currently assumes that for memory starting above 4G, 32-bit devices will + * use a DMA offset. */ static phys_addr_t __init max_zone_phys(unsigned int zone_bits) { - phys_addr_t offset = memblock_start_of_DRAM() & GENMASK_ULL(63, zone_bits); - return min(offset + (1ULL << zone_bits), memblock_end_of_DRAM()); + phys_addr_t base = memblock_start_of_DRAM(); + phys_addr_t offset = base & GENMASK_ULL(63, 32); + s64 zone_size = (1ULL << zone_bits) - (base & DMA_BIT_MASK(32)); + + if (zone_size <= 0) + return 0; + + return min(base + zone_size + offset, memblock_end_of_DRAM()); } static void __init zone_sizes_init(unsigned long min, unsigned long max) @@ -390,6 +397,20 @@ void __init arm64_memblock_init(void) if (IS_ENABLED(CONFIG_ZONE_DMA)) { zone_dma_bits = ARM64_ZONE_DMA_BITS; arm64_dma_phys_limit = max_zone_phys(ARM64_ZONE_DMA_BITS); + + /* + * We don't want to split the 32 bit address space into two DMA + * zones when the target lower physical address range doesn't + * exist. For example, if memory starts at 0x80000000 it's + * pointless to create a ZONE_DMA distinct of ZONE_DMA32 as + * there's no way to access the 30-bit address space. If that's + * the case just expand ZONE_DMA to cover the whole 32-bit + * address space. + */ + if (!arm64_dma_phys_limit) { + zone_dma_bits = 32; + arm64_dma_phys_limit = max_zone_phys(32); + } } if (IS_ENABLED(CONFIG_ZONE_DMA32)) -- 2.28.0
